The Molecular Virulence of Legionnaires' Disease Bacterium.

Legionella pneumophila, a facultative intracellular pathogen is responsible for almost 85% of cases of the legionelloses Legionnaires' disease and Pontiac fever (Dowling et al., 1992; Fields, 1997). These are a consequence of the fact that man has created additional environmental reservoirs for these opportunistic pathogens, providing them with a means by which they can be disseminated in aerosols and exposed to humans (Fields, 1997; Shuman et al., 1998). While both forms of legionellosis are caused by inhalation of adequate numbers of aerosolised legionellae by a susceptible host (Fields 1997), Pontiac fever which is the non pneumonic and less severe kind of illness is not provoked by tissue invasion so it may be caused by strains of Legionella incapable of intracellular multiplication (Dowling et al., 1992). Furthermore, it may involve a contact-dependent cytotoxic effect on the host cell surface (Husmann and Johnson, 1994). These bacteria are characterised as gram negative, aerobic rods (Fields, 1997) and possess a single flagellum with a 47kDa filament subunit, the presence of which predicts virulence (Bosshardt et al., 1997).

In the environment, legionellae are endosymbionts of freshwater protozoa (Byrne and Swanson, 1998), thus they obtain their nutrients from, and multiply in an intracellular environment. At the same time they are ubiquitous in freshwater environments, present at low concentrations in these aquatic reservoirs (Fields, 1997). Therefore the bacteria must have some means to be able to adapt for its survival in different conditions (Byrne and Swanson, 1998). For example, Legionella are susceptible to UV light, and plasmids mediating resistance to UV radiation may be crucial in the pathogen's survival in both natural and manmade water systems (Tully, 1993). Byrne has suggested that when nutrients are abundant in an intracellular environment, L. pneumophila are dedicated to replication and do not express factors correlated with virulence. When nutrients become...